1. Field of the Invention
The present invention relates to broadcasting signal receiving apparatus and pulse counting demodulators, and is directed to an improvement in a broadcasting signal receiving apparatus for receiving selectively frequency-modulated and amplitude-modulated broadcasting signals to obtain reproduced information outputs and further to an improved pulse counting demodulator suitable for use to demodulate a frequency-modulated information signal received by the broadcasting signal receiving apparatus.
2. Description of the Prior Art
In the field of super heterodyne receivers used for receiving frequency-modulated or amplitude-modulated broadcasting signals transmitted from radio broadcasting stations, there has been generally known to employ a digital tuning system wherein, for example, a phase-locked loop (PLL) is utilized in place of an analog tuning system wherein a variable capacitor is used. In the super heterodyne receiver in which the digital tuning system is employed, the phase-locked loop works under the control of a microcomputer to carry out rapid and exact tuning operation and manual adjustments by a user are not necessary so that a tuning portion easy to use is constituted.
The super heterodyne receiver employing the digital tuning system excels in an automatic scanning tuning operation which is convenient for use. Under a condition in which the automatic scanning tuning operation is performed, a receiving frequency provided for receiving selectively frequency-modulated or amplitude-modulated broadcasting signals is successively changed at predetermined regular frequency intervals by the phase-locked loop working under the control of the microcomputer. Then, when there is a frequency-modulated or amplitude-modulated broadcasting signal which corresponds with the receiving frequency, the change in the receiving frequency is temporarily ceased to keep the frequency-modulated or amplitude-modulated broadcasting signal tuned therewith. Such detection of the frequency-modulated or amplitude-modulated broadcasting signal is indicated by an indicator lamp or the like and reproduced information outputs in the form of, for example, audio outputs are obtained from the frequency-modulated or amplitude-modulated broadcasting signal tuning with the receiving frequency are automatically produced.
In such an automatic scanning tuning operation, each of the receiving frequencies obtained successively at predetermined regular frequency intervals corresponds to a carrier frequency of each of frequency-modulated or amplitude-modulated broadcasting signals transmitted from broadcasting stations, respectively. In the case of radio broadcasting signals in Japan, carrier frequencies of frequency-modulated broadcasting signals are arranged at intervals of 100 kHz within a frequency band, for example, from 76 MHz to 90 MHz, which is called an FM band, and carrier frequencies of amplitude-modulated broadcasting signals are arranged at intervals of 9 kHz within a frequency band, for example, from 531 kHz to 1602 kHz, which is called an AM band. Accordingly, the receiving frequency provided for receiving selectively the frequency-modulated broadcasting signals is successively changed to correspond to each of the carrier frequencies arranged at intervals of 100 kHz within the FM band from 76 MHz to 90 MHz and the receiving frequency provided for receiving selectively the amplitude-modulated broadcasting signals is successively changed to correspond to each of the carrier frequencies arranged at intervals of 9 kHz within the AM band from 531 kHz to 1602 kHz.
When the automatic scanning tuning operation is performed in the super heterodyne receiver employing the digital tuning system, it is feared that an erroneous detection in which the frequency-modulated or amplitude-modulated broadcasting signal from a certain broadcasting station which is different from a desirous broadcasting station is undesirably detected is raised. Especially, in the case of the amplitude-modulated radio broadcasting signals, since the frequency interval between each two adjacent carrier frequencies is relatively narrow such as to be, for example, 9 kHz, it is easy to arise that the amplitude-modulated radio broadcasting signal having one of two adjacent carrier frequencies is undesirably detected in place of the amplitude-modulated radio broadcasting signal having the other of two adjacent carrier frequencies which is desired to be detected.
For example, in the case where the receiving frequency is successively changed to increase step by step and correspond to each of the carrier frequencies of the amplitude-modulated radio broadcasting signals under a condition wherein there is a radio broadcasting station transmitting an amplitude-modulated radio broadcasting signal having a carrier frequency of 999 kHz and relatively large signal strength, it is feared that a reception output obtained based on the amplitude-modulated radio broadcasting signal having the carrier frequency of 999 kHz, which is received when the receiving frequency is changed to be 990 kHz, becomes so large that the amplitude-modulated radio broadcasting signal having the carrier frequency of 999 kHz is erroneously detected and as a result the receiving frequency is not changed to reach 999 kHz but caused to keep 990 kHz, so that the automatic scanning tuning operation is ceased.
With intent to avoid the fears mentioned above, there has been previously proposed a system in which, whenever the amplitude-modulated radio broadcasting signal is received with the receiving frequency changed successively to correspond to each of the carrier frequencies of the amplitude-modulated radio broadcasting signals by the phase-locked loop working under the control of the microcomputer, an amplitude-modulated intermediate frequency signal which is obtained by frequency-converting the received amplitude-modulated radio broadcasting signal is supplied to the microcomputer by which the phase-locked loop is controlled and in which the carrier frequency of the amplitude-modulated intermediate frequency signal is detected and it is checked, on the basis of the detected carrier frequency, whether the received amplitude-modulated radio broadcasting signal is desired. The detection of the carrier frequency of the amplitude-modulated intermediate frequency signal in the microcomputer is carried out by counting pulses obtained based on a carrier signal component contained in the amplitude-modulated intermediate frequency signal.
In the case where the pulses obtained based on the carrier signal component contained in the amplitude-modulated intermediate frequency signal are counted in the microcomputer as described above, the amplitude-modulated intermediate frequency signal is subjected to waveform shaping to be shaped into a rectangular waveform signal for obtaining the pulses based on the carrier signal component. This waveform shaping on the amplitude-modulated intermediate frequency signal brings a disadvantage that higher harmonic signals including a signal having a frequency, for example, two or three times higher than the carrier frequency of the amplitude-modulated intermediate frequency signal are produced. The higher harmonic signals thus produced result in such a problem that each of the higher harmonic signals acts on a receiving end for the amplitude-modulated broadcasting signals, such as a receiving antenna coil or the like, as a spurious signal operative to obstruct the amplitude-modulated broadcasting signal from being appropriately received through the receiving end.
For example, when the carrier frequency of the amplitude-modulated intermediate frequency signal is set to be 450 kHz, the higher harmonic signal having a frequency of, for example, 900 kHz (double) or 1350 kHz (three times) results from the waveform shaping to which the amplitude-modulated intermediate frequency signal is subjected to be shaped into the rectangular waveform signal. The higher harmonic signal thus produced with the frequency of 900 kHz or 1350 kHz acts on the receiving end to obstruct the amplitude-modulated broadcasting signal having the carrier frequency of, for example, 900 kHz or 1350 kHz from being appropriately received through the receiving end.